The present invention relates to a manifold system for controlling the operation of safety control valves, particularly in production wells for petroleum products.
In the course of operation of a production well via which a product such as gas or oil is being extracted from an underground deposit and delivered to a pipeline connected to the wellhead and located at or just below the ground surface, there are occasions when it is necessary to halt the flow of the product. Such an operation may be necessary, for example, to permit routine maintenance operations or to prevent spills in the event of an accident or equipment breakdown.
As a result, it has long been the practice in the industry to place at least one shut-off valve in the product flow path, such valve being conventionally located at the wellhead, i.e., essentially at the ground surface. However, location of a shut-off, or safety, valve essentially at the ground surface presents certain drawbacks, particularly since certain types of accidents could damage, or destroy, a valve at that location, in which event the valve could no longer act to block the flow of production fluid.
Therefore, it has more recently become the practice to insert a subsurface shut-off, or safety, valve in the well tubing which conducts the production fluid to the wellhead. Such subsurface valve can be disposed at any depth below the ground surface and is provided with an operating unit connected to systems located at the surface to effect remote control opening and closing of the valve.
The choice of depth for the location of such a subsurface valve is based on a number of considerations, including the depth to which a foreseeable accident occurring at ground level could damage such a valve, external conditions relating, for example, to the climate in which the well is located, and legal requirements. Consideration must also be given to the fact that the cost of installing, servicing, or replacing such a valve increases as a function of the depth at which the valve is to be located.
For example, in the case of a production well located in the North Slope of Alaska, where the permafrost layer extends to depths in excess of 2,000 feet, both State and Federal laws require that the subsurface safety valve, which is ordinarily a ball valve, be located below the permafrost level, and thus at a depth in excess of 2,000 feet. Since repair or replacement of a valve located at such a depth can be expected to be enormously expensive, particularly in the Arctic where, in view of the severe weather conditions, such replacement could conceivably cost $1,000,000 or more, it is important that steps be taken to avoid the need for servicing or replacing the subsurface valve.
For these reasons, it is desirable to dispose in the product flow path two valves, one located at the required level below the surface and one located at the surface, and to operate the valves in sequence in a manner to prevent the subsurface valve from opening or closing against high dynamic pressure loads or high flow rates.
In order to block the flow of production fluid from such a well in the event of an accident, it is a general practice to provide monitoring equipment which senses certain conditions, such as the pressure or rate of flow of product at the wellhead, the temperature of the environment surrounding the wellhead, etc., and to connect this monitoring equipment to effect closing of the safety valve, or valves, upon the occurrence of a condition indicating that an accident or malfunction has taken place. Of course, when such a condition is sensed, it is desirable that the safety valve, or valves, close as rapidly as possible.
In many cases, it is also desirable that the response of the system to an unsafe condition indication, or the sensitivity of the system be adjustable to compensate for changes in external conditions which can influence the operation of the system, or for unavoidable changes in the operating characteristics of various components of the system. It may also be desirable to be able to vary the speed of response of the system to an indication of an unsafe condition if, for example, external factors make it undesirable to close the safety valve, or valves, in the shortest time that the capabilities of the system permit.
On the other hand, it is equally desirable that the system which acts to close the valve, or valves, in response to the indication of an unsafe condition be as simple as possible since the reliability of any system is directly related to its structural simplicity.